1. Field of the Invention
The present invention relates to an illumination optical apparatus and method suitably applicable to exposure apparatus used in the photolithography process for fabricating for example semiconductor devices, liquid crystal display devices or thin-film magnetic heads.
2. Related Background Art
The photolithography process for fabricating for example the semiconductor devices employs a projection exposure apparatus having an illumination optical system for irradiating illumination light from a light source onto a photomask or a reticle (which will be collectively referred to as a reticle) and a projection optical system for projecting an image of a pattern on the reticle onto a substrate (semiconductor wafer, glass plate, etc.) coated with a photosensitive material (photoresist). A recent trend is to use reduction projection exposure apparatus of the step-and-repeat method, i.e., so-called steppers, as disclosed for example in U.S. Pat. No. 4,699,515.
In order to enhance illuminance uniformity on the reticle, the projection exposure apparatus uses a fly-eye type optical integrator (fly-eye lens) for example as disclosed in U.S. Pat. No. 4,619,508 or U.S. Pat. No. 4,668,077. The exit plane of fly-eye lens is a Fourier transform plane for the pattern surface of reticle in the illumination optical system. A surface illuminant image (an aggregation of point sources corresponding to associated lens elements constituting the fly-eye lens) is formed at this plane.
Further, U.S. Pat. No. 4,497,013 or U.S. Pat. No. 4,497,015 discloses such an arrangement that two fly-eye lenses are aligned along the optical axis of illumination optical system to greatly increase the number of point sources whereby the illuminance uniformity is improved on the reticle. Also, U.S. Pat. No. 4,918,583 discloses an arrangement using a rod-type optical integrator together with the fly-eye lens to improve the illuminance uniformity, and U.S. Pat. No. 5,153,773 discloses an arrangement in which a plurality of beams are made obliquely incident into the fly-eye lens to increase the number of point sources whereby the illuminance uniformity is improved. If a high-power laser such as an excimer laser is used and when a laser beam therefrom is focused on the exit plane of fly-eye lens to form point sources there, each lens element could be damaged thereby. Thus, U.S. Pat. No. 4,939,630 suggests such an arrangement that the point sources are formed at positions apart from the exit plane of fly-eye lens.
Incidentally, the entrance plane of fly-eye lens is conjugate with the pattern-formed surface of reticle. Because of this, a light quantity loss of illumination light becomes minimum when the entrance plane of each lens element is similar to an effective pattern area of reticle (a maximum area of pattern to be projected onto the substrate). Actually, the effective pattern area of reticle is often rectangular, because chip patterns of LSI or the like are rectangular. Therefore, the shape of the entrance plane of each lens element in the fly-eye lens is rectangular (of course, the shape of the exit plane is also rectangular).
An image field of the projection optical system used in steppers is rectangular but considerably close to square, i.e., rectangular with the vertical or longitudinal length being not so different from the horizontal or transversal length. Accordingly, the shape of the entrance plane of each lens element in the fly-eye lens is also rectangular but close to square. On the other hand, there are steppers with a fly-eye lens having square lens elements in cross section, because the effective pattern area itself of the reticle is square.
Accordingly, the conventional fly-eye lenses for steppers are formed such that lens elements each with square or almost-square-rectangular cross section are arranged vertically and horizontally. Light source images are formed on or near the exit plane of respective lens elements, so that the light source images are formed as an aggregation arranged in a grid pattern at same or slightly different longitudinal and transversal pitches.
Since the degree of integration for semiconductor devices is becoming increasingly higher these days, it is required to further enhance the resolution (i.e., resolving paver) of a pattern projected onto the substrate. To meet the requirement, the numerical aperture of the projection optical system could be increased to improve the resolution, but it is not practical because the depth of focus becomes too shallow. Then there is a proposition of the modified light source method in which the shape of secondary light sources (or tertiary light sources, etc.) is modified in various ways in the illumination optical system to improve the resolution or the depth of focus of the projection optical system. In the modified light source method one of apertures of various shapes is set on the exit plane of fly-eye lens, that is, on the plane in a relation of Fourier transform with the reticle pattern. Further, the annular illumination method employs an aperture for making the shape of secondary (or tertiary) light sources annular.
As for the steppers, the longitudinal pitch is not so different from the transversal pitch for point light sources (secondary or tertiary light sources) formed on or near the exit plane of fly-eye lens. There are, however, recent propositions of a scanning projection exposure apparatus with an aspect (length-to-width) ratio of the effective pattern region on the reticle being greatly offset from 1:1, as disclosed in U.S. Pat. No. 4,747,678, U.S. Pat. No. 4,924,257 or U.S. Pat. No. 5,194,893. A stepper can be so arranged that the effective pattern area is 100 mm square on a reticle, that is, a good-image range in the image field of the projection optical system (with projection magnification of 1) is 141 (=2.sup.1/2 .times.100) mm in diameter (.phi.). In contrast, in case a scanning projection exposure apparatus has the same good-image range of the projection optical system, i.e., .phi.=141 mm, an illumination area on the reticle has the width in the scanning direction of 44.7 mm and the width in the non-scanning direction perpendicular to the scanning direction, of 134.2 mm. That is, the aspect ratio of the illumination area is approximately 1:3. Accordingly, an area of chip pattern transferable onto the substrate by one scanning exposure is 134.2 mmx(maximum movement stroke in the scanning direction) on the reticle. This permits the scanning projection exposure apparatus to form a considerably large chip pattern as compared with the steppers.
However, the scanning projection exposure apparatus must be so arranged, in order to decrease a loss in illumination light quantity, that the cross section of each lens element in the fly-eye lens is rectangular with an aspect ratio of 1:3, matching with the shape of the illumination area on the reticle. The lens elements of such cross section cause no problems in respect of machining or in respect of light quantity. However, point light sources formed on the exit plane of fly-eye lens have a longitudinal pitch three times larger than the transversal pitch. Such a large difference between the longitudinal (scanning direction) pitch and the transversal (non-scanning direction) pitch of point sources could cause a problem that imaging properties (exposure amount, resolution, depth of focus, etc.) in the scanning direction for an image of reticle pattern are different from those in the non-scanning direction. Further, if the annular illumination method or the modified light source method is employed, the above problem becomes more pronounced because an aperture stop shields illumination light from specific lens elements in the fly-eye lens so as to decrease the number of point sources.
Even if the annular illumination method or the modified light source method is applied to the projection exposure apparatus in which the effective pattern area on the reticle is rectangular with the aspect ratio of approximately 1:1, such as the steppers, there could occur such a problem that the imaging properties are different from each other for example between two orthogonal directions. The present applicant has proposed a method of improvement in U.S. application Ser. No. 020,775 (filed Feb. 22, 1993, corresponding to U.S. Pat. No. 5,335,044 issued Aug. 2, 1997). This method of improvement is, however, effective up to about 1:1.5 of the aspect ratio of effective pattern area, but cannot be so effective when the aspect ratio exceeds for example about 1:2.
In case a laser beam is used as the illumination light, light source images formed on the exit plane of lens elements in the fly-eye lens are almost point sources, which necessitates no consideration on a light quantity loss. In contrast, if, for example, the g line or i line from a super-high pressure mercury lamp is used as the illumination light, the light source images become a kind of surface illuminant. Then, with use of a fly-eye lens including a bundle of lens elements rectangular in cross section with the illumination light from the mercury lamp, such a problem could occur that the illumination light is eclipsed in the transverse direction of each lens element whereby a sufficient quantity of light cannot be attained.
Hence, in case of the fly-eye lens including a bundle of lens elements rectangular in cross section being used, a plurality of light sources are arranged along the longitudinal direction of lens elements and beams from the light sources are guided into the fly-eye lens in mutually different directions, for example as disclosed in Japanese Laid-open Pat. No. Application No. 5-45605. Then there are a plurality of images corresponding to the light sources formed along the longitudinal direction on the exit plane of a lens element in the fly-eye lens, improving the illuminance uniformity and the illumination power (illuminance) on the reticle.
The arrangement as disclosed in the above Japanese application, however, has such a problem that if the plurality of light sources have different illumination powers illuminance unevenness occurs on the exit plane of fly-eye lens so as to lower the illuminance uniformity on the reticle. Also, in case one out of the plurality of light sources is off (e.g., burned out), the illuminance unevenness on the reticle would become out of a permissible range. Further, the apparatus must be stopped during exchange of the light source, which causes a problem of a great decrease in throughput.
Further, a recent report showed an improvement in depth of focus or in resolution by optimizing the a value (a value of reticle-side numerical aperture of illumination optical system/reticle-side numerical aperture of projection optical system) of illumination optical system in accordance with a reticle pattern. It becomes thus important to set the .sigma. value of illumination optical system accurately to its optimum value. The .sigma. value of illumination optical system is determined according to the diameter of illuminance distribution of light source images on the exit plane of fly-eye lens (which is a plane conjugate with the pupil plane of projection optical system). Because of a nonuniform illuminance distribution of light source images, such a problem would occur that the substantial .sigma. value determined from the illuminance distribution (effective .sigma. value) is different from the design .sigma. value (nominal .sigma. value).